1. Field of the Invention
The present invention relates to the field of screen printing presses and, in particular, to a press for printing heat transfers on a substrate.
2. Discussion of the Prior Art
Heat transfers are preprinted artwork or the like that are printed in a mirror image or facedown orientation on a specialized substrate. The ink used in the printing of heat transfers must be semi-cured or gelled after each color is printed and must be printed with a heavy deposit to ensure the proper transfer from the substrate to the garment and to ensure a durable, finished product. To apply a transfer to an article of clothing or the like, the transfer is placed against the article and a heated iron or press is used to heat the transfer while pressing it against the article until the ink of the artwork releases from the substrate and melts into the article. When the substrate is removed, the artwork remains on the article.
It is conventional to employ a multi-station, turntable-type printing press for printing transfers. Such a conventional press includes an upstanding frame including a center post about which a loading and unloading station, a plurality of printing stations, and a plurality of drying stations are disposed. Silk screen printing assemblies are supported on the frame at each printing station, and a conventional drying means is disposed at each drying station. A turntable is supported on the frame for rotation about the center post, and includes a plurality of vacuum beds that rotate with the turntable and on which the transfers are supported.
Each unprinted substrate is placed on the bed positioned at the loading/unloading station and is held in place on the bed by negative pressure drawn through orifices in the bed so that the substrate is rotated with the turntable as a different color of the transfer is printed on the substrate at each printing station and cured at each curing station. Once printing of the transfer is complete, the bed carrying the substrate is rotated to the loading/unloading station and the negative pressure is relieved so that the substrate carrying the completed transfer may be removed from the bed.
Heat is commonly used to dry or cure the ink of the transfers subsequent to each printing operation. The temperature of the substrate must be kept within a narrow range suitable for curing. If the temperature of the substrate gets too hot, the substrate may scorch or the ink may over-gel, ruining the transfer. If the temperature is too cool, the ink will not cure and may stick to subsequent printing assemblies. Further, within this range of temperatures at which the transfers must be cured, different temperatures work better for different colors and types of inks, and it is essential that the curing operation be tailored to the particular color or type of ink applied in order to obtain proper curing of each color on the transfers.
Because in conventional printing presses, the vacuum beds are rotated with the substrates through the sequential printing and curing operations, heat builds up in the beds and substrates as the beds rotate through each curing operation. If the heat build-up is severe, the ink will continue to be heated after the drying operations, causing the ink to over-gel and ruining the transfer or remain semi-molten, causing it to stick to subsequent screens. Thus, it is necessary to slow or stop the printing process in order to permit the beds to cool after each curing operation and prior to subsequent printing operations. Alternately, other means are known for insulating the substrates from the vacuum beds so that heat carried by the beds is not conducted to the inks of the transfer.
In addition to the heat transfer problems presented by the use of vacuum beds on a rotatable turntable, the use of such a construction also requires that rotation of the turntable be repeatedly started and stopped as the beds are rotated from one station to the next. However, the weight of the beds on the turntable impedes the ability of a conventional indexing mechanisms from quickly and accurately rotating the turntable to each desired position relative to the printing assemblies.
Also, prior art presses use infrared for ink curing. The infrared heat cannot be controlled with the desired level of precision. As a result, the ink can be overheated.
The high viscosity of transfer printing inks requires the screen to be lifted progressively during the print stroke thereby effecting a gradual separation or peel between the screen and the substrate behind the squeegee. Screens gradually lose tension because of the constant flexing during printing operations. A low tension screen must be lifted higher and more quickly than one with greater tension if proper peeling action is to occur. In prior art printing heads, the screen is linked with the squeegee mechanism for lifting the screen as the squeegee travels through the print stroke. If the operator does not properly adjust the lifting mechanism, the print quality can be adversely affected.
Finally, turntable-type transfer printing presses include structural members for holding the print heads. These structural members are positioned over the screen and inhibit operator access for adjustments and ink addition.